This invention relates to improvements in circular saws and more particularly to improvements in high speed circular saws suitable for various cutting operations in modern saw mills. In order to achieve maximum operating efficiency, the trend has been to use increased saw speeds. Also, the thickness of the saw plate has been considerably reduced to decrease the amount of waste. The saw plate can no longer be considered a stiff member. Indeed, to an extent, it can be considered analogous to a semiflexible membrane connecting the arbor, or shaft, with the saw teeth.
With higher speeds saw plate vibration is a major problem and is a constraint on the maximum available cutting speed. Excessive saw plate vibration tends to cause overheating, crooked cuts, a shortening of the saw's life and noisy operating conditions. Also, and more importantly, excessive vibration can cause hazardous catastrophic failures of the saw.
Tests show that saw vibration manifests itself in two significant forms. In the first form standing wave patterns tend to establish themselves on the rotating saw plate. These are referred to as nodal vibration patterns. The second form involves tooth-to-tooth harmonic vibrations and the related vibrations of the material being cut. These will be more fully understood in the discussions which follow.
For reasons which I cannot explain, except perhaps because of habit or man's inherent sense of symmetry, circular saws have been traditionally designed to have even numbers of teeth. For example, saws having 12, 16, 18, 24, 36 and higher even numbers of teeth are common. It is also traditional to uniformly space the teeth around the periphery of the saw plate. The various even numbers of teeth commonly employed in circular saws are divisible by several factors. For example:
Twelve teeth are divisible by 2, 3, 4, or 6,
Sixteen teeth are divisible by 2, 4, or 8,
Eighteen teeth are divisible by 2, 3, 6, or 9,
Twenty-four teeth are divisible by 2, 3, 4, 6, 8, or 12, and
Thirty-six teeth are divisible by 2, 3, 4, 6, or 12.
The foregoing prior art circular saws are prone to incur both harmonic and nodal vibration problems. The evenly spaced saw teeth impact the work piece at equally spaced time intervals so that there is a tendency to set up harmonic vibrations in the saw plate, in the saw teeth and in the work piece, or material, being cut. Also, the divisible nature of the number of teeth in the saw makes likely the establishment of nodal vibration patterns resulting in a standing, or stationary, wave in the saw having a period which corresponds to a factor of the total number of teeth.
There have been many attempts to reduce saw vibration. These have met with only limited success. Thus, for example, the varying of tooth spacing within narrowly repetitious patterns has been proposed by Owen, et al, in patent U.S. Pat. No. 2,718,245 and by Blum, in U.S. Pat. No. 2,351,737. Also, varying the height of alternate teeth has been proposed by Soderstrom in U.S. Pat. No. 3,878,747 and by Owen, et al, above. Also, systematic variations of the side bevels in adjacent groupings of teeth has been proposed by Edmiston in U.S. Pat. No. 2,770,267. In addition to the foregoing, investigators have suggested the use of asymetrical edge slots to dampen nodal vibrations. However, none of the foregoing prior art recommends a satisfactory solution of the problem of dampening nodal vibration while minimizing tooth-to-tooth harmonic vibration and maintaining a substantially continuous circular saw.